You're doing it right. But gmail won't let you.
To quote the answer linked to....
Google rewrites the From and Reply-To headers in messages you send via it's SMTP service to values which relate to your gmail account.
The SMTP feature of gmail isn't intended to be an open or relay service. If it allowed any values for the From header, it would significantly dilute Google's standing with spam services, as there would be no way to verify the credentials of the sender.